The present invention relates to magnetic head compensators and, in particular, to equipment capable of handling media containing information stored magnetically at various wavelengths.
Known mass storage media store digital data on a magnetic tape or disk in a high density bit pattern. As this technology advances, improved heads and recording techniques allow higher density recording of the bit patterns, at times increasing the density by a factor of two or more. With certain recording channel codes such as that used in the tape drive industry, standard HI/TC products (and incidentally, in the IBM 3480 tape drive), the all-zeroes pattern, recorded at double the frequency of the all-ones pattern, is deliberately suppressed in the readback process. However, if a new read head is designed for a tape drive which is to operate, for example, at double the bit density (hence frequency) of the original, but still must also be capable of reading tapes previously recorded on the original lower density equipment, a serious problem arises. The same frequency (all-zeroes) that was suppressed in the original readback process now represents the all-ones pattern, which must not be suppressed when the head is reading the double-density tape. Yet that frequency must be suppressed when the head is reading the original lower density tapes. Similar statements could be made if instead the density were to be tripled, quadrupled, etc.
A higher density normally requires a smaller gap length on the magnetic heads that read from and write to the tape or disk. It is well known that the output from a magnetic head reading the recorded information varies as function of sin(z)/z, where z is inversely proportional to wavelength and directly proportional to the gap length times 1.12 .pi..
The foregoing includes a factor to correct the physical gap to the functional gap, according to the well-known Westmijze paper.
In principle, a filter can be switched into the readback path of the tape drive, which would alter the overall transfer function of the new head design and read channel to behave as the original head and readback path when it is desired to read tapes written at the original density. This filter in tandem with the new read head reproduce the transfer function of the original head. However, in general, filters having the desired amplitude response have an improper phase response and vice versa.
Transversal filters, a form of "universal" filter, are often represented and implemented by a delay line of length 2NT with (2N+1) equally spaced taps. The outputs of the taps are scaled to A.sub.n and A.sub.-n and are summed in an output summer. The output of the filter is delayed by NT, half the delay line length. If each A.sub.n is set equal to A.sub.-n, one gets zero phase shift.
Accordingly, there is a need for a technique for enabling a drive to read various media recorded at various wavelengths in a simple way without introducing phase distortions that can adversely affect performance.